This invention relates to a preservative composition for porous inorganic substrates and to the protection of porous inorganic substrates from damage by salt water, gaseous pollutants, graffiti, and erosion.
The erosion of ancient stone art objects, or of modern stone and concrete structures, by salt water, rain, or gaseous pollutants, alone or in combination, has become a problem of substantial proportions owing to high concentrations of acidic gases in the atmosphere. The major sources of these pollutant materials are automobiles, power plants and industrial installations, which together pour thousands of tons of soot and harmful chemicals, particularly sulfur dioxide and nitrogen oxides, into the air each day. These gases, which wash stone and concrete buildings, including marble and limestone art objects and ancient cathedrals, damage the structures by causing gradual dissolution of carbonate minerals.
Another problem of modern day civilization is the tendency of irresponsible members of the public to inscribe graffiti on public buildings. It is apparent that a method of protecting the buildings from such graffiti would be highly desirable, particularly if the protective system also afforded protection against the effects of atmospheric pollution and moisture.
Although various coatings have been used on stone work and concrete constructions, generally these compositions have been inadequate for protecting the stone or concrete against damage by atmospheric pollutants and graffiti, usually because the coating did not permit the structures to "breathe," that is, the exchange vapor, carbon dioxide and other gases, with the air.
In the road maintenance art, a long-standing problem is the damage caused to concrete roadbeds, road dividers, curbs and sidewalks by materials used in deicing operations, e.g., rock salt (sodium chloride), calcium chloride or urea. Protective coatings for concrete or Portland cement-based structures must be such that the structure can recieve water vapor, carbon dioxide, etc. That is, concrete structures must breathe. On the other hand, the concrete structure must be protected from chemicals used for deicing, which would otherwise leach into the cement and tend to cause its disintegration. For example, it is necessary to replace the curbs in the Washington, D.C., area on the average of every third year owing to combined damage by run off from salts used to melt snow and the accumulation of salted snow piled on the curbs by snow plows. The replacement rate varies according to the amount of salts used to melt snow, but it will be appreciated that the relative economy of salt compared to other methods mandates its use in many sections of the country.
It is known in the art of protecting and preserving stone objects to use a mixture containing barium or strontium ions in order to cause the precipitation of barium or strontium sulfate in objects made from calcareous stones, such as limestone and sandstone, to effect consolidation of the stone. However, as indicated in Sayre (U.S. Pat. No. 3,704,159), the technique is not as simple as it appears, because of the need to deposit more than a few crystals of barium or strontium sulfate at the surface of the stone object. An additional drawback of this method is that several of the known methods require more than one process step and are therefore expensive in terms of labor. Moreover, the use of barium or strontium salts as a binder does not protect the stone objects against graffiti.
Berger, in U.S. Pat. No. 3,438,804, teaches that polyisobutenylsuccinic anhydride can be used as a preservative for normally water-absorbent materials, such as concrete highways and bridges. The materials are applied from a water-hexane emulsion to produce a water-repellant coating which is permeable to water and gases. However, the hydrophobic film is not permanent, in that there is a significant increase of water-absorbency in treated samples of concrete after seven weeks' outdoors testing.
Dreher et al (U.S. Pat. No. 3,850,661) suggest the use of an epoxide-based composition for impregnating porous inorganic substrates, including roadways. However, the composition has a very short pot life, of the order of eight hours or shorter, so that the composition is difficult to handle.
Evans (U.S. Pat. No. 3,196,122) teaches the preparation of acid-resistant cementitious material, in which a polymeric latex and a hydraulic cement are the basic ingredients. Although this material can be used for patching and overcoating masonry structures, it appears to provide no protection against graffiti or salt water.
Lal Gauri (U.S. Pat. No. 3,795,533) teaches preserving and strengthening wood or stone by a treatment involving sequential impregnation with a plurality of solvent-resin solutions. The deficiency of this method over a one-step process will be readily apparent.
Thus, there is a continuing need for materials which protect porous inorganic substrates from the deleterious effects of salt water, acidic gases, erosion, graffiti and the like and which are simple to apply and have a reasonable working life.